No. 3.] DEVELOPMENT OF MARINE SPONGES. 333 
of the period, whatever it may be, the larvae become sluggish 
and lie about on their sides on the bottom of the dish. They 
then attach, the columnar ectoderm cells are transformed into 
flat cells, and the body of the larva flattens out into a round 
cake-like mass. The distinction between the pigmented and 
unpigmented portions of the body is entirely lost, the whole 
surface becoming red. In Pl. XXI, Fig. 84, is shown a surface 
view of recently attached sponge, and in Pl. XXII, Fig. 92, one- 
half of a vertical section through the same. The long spicules, 
which form a bundle in the unpigmented end of the larva, 
become scattered irregularly through the body. In attaching, 
almost all the larvae I have watched have stuck fast by their 
sides and not by one end. Such a larva just attached is shown 
in Pl. XXI, Fig. 85. Its outline still recalls the outline of the 
swimming larva (Fig. 80), it not yet having assumed the 
circular shape of Fig. 84. The body is solid; the ectoderm is 
entirely composed of flat and very thin cells; and the unpig- 
mented or spicular pole of the larva (sf. #.) can still be 
identified both by the absence of pigment and the presence 
of the long spicules. The line of demarcation between the 
pigmented and unpigmented regions is not a sharp one, as it 
was in the swimming larva, and the spicules are no longer 
arranged in a bundle, but have begun to scatter about, though 
as yet they are still confined to one end of the sponge. 
The attachment may take place obliquely, so as to bring the 
spicular pole on the upper surface of the metamorphosed larva, 
though near the periphery. In a few cases I have seen the 
attachment take place by the non-spicular pole, a little obliquely, 
to be sure, as is shown in the surface view, Fig. 82. In this 
larva the spicular pole was pulled in to such an extent, that 
at first sight it looked like an opening leading into the interior, 
though as a matter of fact it was nothing of the kind. When 
the attachment takes place by the end, as in Fig. 82, the 
spicular pole comes to occupy a more or less central position 
on the upper surface of the metamorphosed larva. In Pl. XXI, 
Fig. 83, is given a vertical section through a little sponge, 
which must have attached by the non-spicular poie, for on the 
upper surface and more or less in the centre is found the 
